1. Field of the Invention
This invention relates to a metal gasket adapted to be interposed between opposed surfaces of parts of a multicylinder engine and seal a clearance between the parts.
2. Description of the Prior Art
A metal gasket has heretofore been used to seal a clearance between opposed surfaces of a cylinder head and a cylinder block of an engine, and various kinds of metal gaskets have been developed. A recently used engine has demanded a higher output level and smaller weight, and, with the intention of meeting the demand, there is a tendency for a cylinder head and a cylinder block to be made of an aluminum material of a smaller specific gravity instead of conventionally used steel and a cast iron of a larger specific gravity. Since an aluminum structural member of an engine has smaller weight but a lower rigidity, the relative displacement of a cylinder head and a cylinder block tends to increase during an operation of the engine.
A metal gasket is provided with beads in the vicinity of circumferences of cylinder bores and passage holes for water and oil, which beads form annular elastic contact portions with respect to the opposed surfaces of a cylinder head and a cylinder block when the metal gasket is fixed by tightening the cylinder head and cylinder block by bolts, whereby a clearance between the opposed surfaces of the cylinder head and cylinder block is sealed.
In the case of a cylinder head metal gasket, repeated stress, i.e. mechanical stress and thermal stress are exerted thereon since a clearance between a cylinder head and a cylinder block increases and decreases repeatedly during a combustion cycle of the engine. The engine load fluctuation stress occurs at a high level in the portions of a cylinder block and a cylinder head which have the lowest rigidity. Consequently, permanent set in fatigue and cracks occur in the beads formed on a beaded base plate to cause the sealing performance of the metal gasket to be deteriorated.
A conventional cylinder head metal gasket disclosed in Japanese Patent Laid-Open No. 61506/1996 is formed by laminating in order a beaded base plate comprising an elastic metal plate provided with beads, which extend along the circumferences of bores, a regulating plate comprising an elastic metal plate provided with beads, which extend along the circumferences of bores, and a grommet plate provided with holes. In this gasket, an inner circumferential edge of a bore of the regulating plate is held fixedly by the grommet plate, and the beads on the beaded base plate which extend between bores comprise full beads, the same beads which extend around the other portions of the bores comprising half beads. The beads on the regulating plate comprise full beads with respect to the portions thereof which are around the whole circumferences of the bores, and the portions of the beads on the beaded base plate which are between the bores are superposed on the corresponding portions of the beads on the regulating plate.
In a metal gasket, concentrating a surface pressure, that is, a face-to-face pressure properly on the regions around bores, and securing an average face-to-face pressure condition in which a required face-to-face pressure is secured around water holes and oil holes serve to improve the sealability of the gasket, and are desirable for balancing the face-to-face pressure.
However, in a conventional metal gasket, the face-to-face pressure is concentrated excessively on the grommet portions, and a required face-to-face pressure cannot be secured around the water holes and oil holes. Therefore, it becomes difficult to secure an average face-to-face pressure condition, so that an excellent sealed condition cannot be maintained. When a face-to-face pressure is concentrated on beaded portions of a metal gasket, the balance of a face-to-face pressure is deteriorated, and the sealability of the gasket lowers.
When a metal gasket having a beaded base plate with beads projected from an outer surface thereof is tightened between a cylinder head and a cylinder block, the projecting surfaces of the beads directly contact the cylinder head or cylinder block, and only the regions which the projecting surfaces of the beads contact generate a high linear face-to-face pressure. Consequently, impressions and breakage occur on and in the cylinder block or cylinder head which the projecting surfaces of the beads contact, and, moreover, the sealing performance is deteriorated.